Water curable, polyurethane (or urethane) compositions are known. Desirably, these are storage stable but readily and controllably cure or otherwise solidify with the use of water-containing curing agent(s) that are reactive with isocyanate moieties of the resin. The water may be in the form of liquid water vapor, steam, and/or the like. It may also be in a latex, emulsion, dispersion, slurry, gel, or the like. The cured compositions are waterproof, chemically resistant, elastomeric, readily cure under a wide range of temperature conditions with minimal shrinkage or expansion due to foaming. The compositions may be used as glue and/or applied, coated, trowelled, poured, shaped, sprayed, injected, rolled, brushed, or otherwise used to form structures of any desired thickness, including coatings, membranes, skid-resistant surfaces, plugs, gaskets, caulking, crack fillers, seals, encapsulation, three dimensional structures, and the like.
Embodiments of water curable polyurethane compositions are commercially available. These have advantageously been used as base and/or top coatings over commercial and residential floors or other walkways, roofing, plumbing, piping, columns or other architectural items, zoo enclosures, decking in marine environments, exterior walkways, and the like. These products can be applied as thick or as thin as desired and will cure controllably and consistently throughout using water as a curing agent.
A typical water curing composition has at least two parts. The first part includes an isocyanate-functional resin (also referred to as a prepolymer) and one or more additives that facilitate the processability, shelf life, handling, and/or performance (before or after curing) of the composition. The second part incorporates the curing agent, which generally includes water. The water may be supplied as steam, ambient humidity, vapor, a solution, an emulsion, a dispersion, a latex, or the like. A large, stoichiometric excess of water may be used. For example, the commercial embodiments noted above may be cured in this manner. U.S. Pat. No. 4,426,488 also describes a water curable polyurethane composition that can be cured in this manner. At the time of use, the first part containing the resin and the second part containing the curing agent are caused and/or allowed to interact. While the composition still has fluid properties, it is sprayed, coated, brushed, trowelled, poured, squeegeed, injected, or otherwise used in the desired manner. Thereafter, the composition cures and solidifies in due course.
Conventional NCO functional resins used in these compositions are generally formed by reacting a polyol component with a stoichiometric excess of a polyisocyanate component. Due to its favorable reactivity and commercial availability, toluene diisocyanate (TDI) has been widely used as the predominant constituent of the polyisocyanate component. However, TDI has a high vapor pressure. The content of TDI incorporated into such resins generally must be restricted for safety and environmental reasons. Safety and environmental concerns arise due to the practical realities of manufacturing NCO functional resins from polyol(s) and a stoichiometric excess of polyisocyanate(s). When excess polyisocyanate component and a polyol component are reacted, there is a strong likelihood that there will be at least some leftover, unreacted isocyanate functional reactants. Any leftover reactants that are relatively volatile, such as TDI if present, will tend to outgas to some degree from the composition during use and perhaps even after curing if curing does not go to completion.
Because emissions of volatile organic compounds such as volatile isocyanates are closely regulated, precautions are taken with respect to conventional compositions such as those incorporating TDI. Firstly, to minimize the amount of leftover diisocyanate such as TDI that might remain, the amount of the monomer in the formulation is restricted. Further, the reaction between such monomer and the polyol component is carefully carried out as far to completion as practical. Secondly, at the time of use, the compositions are used with appropriate precautions until cured. The need to carry the reaction so far to completion tends to increase manufacturing costs. The need to limit the isocyanate content of a resin also tends to limit beneficial characteristics of the resin whose quality tends to increase as a function of increasing urethane/urea content.
Recently, water-curable compositions have been developed in which a relatively low vapor pressure, NCO-functional compounds are substituted for all or a portion of the more conventional TDI. For example, assignee's co-pending U.S. provisional application titled WATER CURABLE POLYURETHANE COMPOSITIONS AND USES THEREOF in the name of L. Gupta, filed Apr. 9, 2002, and having U.S. provisional application Ser. No. 60/371,253, incorporated herein by reference in its entirety, describes water curable compositions whose NCO-functional resin component(s) incorporate MDI-based compounds. The vapor pressure, and hence volatility, of these compounds is extremely low. The use of these compositions is very desirable in those applications in which emissions of volatile organic compounds are desirably reduced.
Water curing of the water-induced urethane compositions can occur through external or internal mechanisms, as desired. External curing relies upon moisture in the ambient to effect crosslinking of the isocyanate functional resin. External curing has limitations. Inasmuch as the temperature and ambient humidity are not easily controlled in all instances, external curing can be unpredictable and/or unreliable.
Additionally, it is difficult to form relatively thicker coatings or other structures because ambient moisture must be allowed to diffuse or otherwise migrate throughout the material for curing to take place.
In contrast to external curing, internal curing involves mixing a substantial stoichiometric excess of water and any other desired constituents of the curing agent with the first part at the time of use. As a consequence, the compositions will cure throughout regardless of the thickness of the material or the ambient humidity. Internal curing also ensures that a substantial stoichiometric excess of water is present during curing, which is desirable as a handling aid. Internal curing is reliable and consistent.
While the exact nature of the curing reaction(s) is not known with certainty, it is generally believed that the water reacts with the NCO functionality on the resin to form urea linkages. A by-product of this reaction is carbon dioxide, CO2 If not appropriately controlled in some fashion, the evolution of the CO2 can cause excessive foaming, blistering, deleterious bubble formation, or otherwise impair the quality of the resultant cured material. To help control foaming, CO2 scavengers are used. These generally include one or more compounds that chemically and/or physically interact with the CO2 and/or other aspects of the composition in some fashion so as to alleviate the degree of foaming that would otherwise occur.
Examples of CO2 scavengers that are believed to chemically interact with the CO2 include alkaline compounds such as magnesium oxide, magnesium hydroxide, calcium hydroxide, and calcium oxide.
Even when using one or more CO2 scavengers, foaming may still be difficult to control. In particular, increasing the NCO content of the resin (which includes the NCO functionality of not just the resin itself but also any unreacted monomer and/or reaction by-products, if any) tends to exacerbate foaming. Thus, although it is generally desirable in some circumstances to formulate with higher NCO content, doing so is not always practically feasible. It is thus more difficult to control foaming with increasing NCO content. Similarly, using NCO functional compounds that have greater rates of reaction with moisture also tend to exacerbate foaming. Thus, although it may be desirable to use such compounds in some circumstances, doing so might not always be practically feasible.
Viscosity characteristics of moisture curable compositions are important in at least two respects. First, the viscosity characteristics of the one or more admixture(s) used to prepare the compositions are desirably low enough to facilitate processing at the desired processing temperatures. If the viscosity is too high, the admixtures are more difficult to handle. In the past, ingredients comprising one or more plasticizers have been used to help reduce admixture viscosity. However, admixtures comprising relatively higher NCO content still tend to have higher viscosity than might be desired at the processing stages of manufacture even when a plasticizer is incorporated into such compositions in accordance with conventional practices.
Secondly, after the compositions are made and packaged, the viscosity characteristics of the uncured compositions should be sufficiently stable over time so that the compositions have good shelf life. However, the present inventors have determined that moisture curable compositions with higher NCO content tend to have viscosity stability issues. Merely reducing the NCO content down to levels at which sufficient stability is achieved is not always practical inasmuch as the resultant modified formulation may yield cured materials with poor hardness characteristics depending upon the particular end use application. Merely reducing the NCO content might not even solve the stability problem. The present inventors have also determined that compositions including NCO-functional resins derived from MDI-based materials have a greater tendency to have unstable viscosity characteristics as compared to conventional compositions based upon TDI.
A key technical challenge has been to uncover a way to use relatively higher NCO content while still maintaining stable viscosity characteristics. Another key technical challenge has been how to formulate water-curable compositions with stable viscosity characteristics when those compositions include NCO-functional resins derived from MDI materials.